The human spinal column is prone to diseases or disorders that produce disruption of the normal alignment of the spine. Frequently, treatment of spinal disorders involves spinal stabilization, for example, by immobilization of the affected vertebral joint(s). One spinal stabilization technique includes a surgical process wherein implants are attached to the spinal vertebrae and connected with spinal rods. In particular, a combination of bone screw arrangements and connecting rods are used to provide a stabilizing construct secured to the spinal vertebrae for the purpose of stabilizing and/or adjusting spinal alignment.
Conventional bone screw arrangements generally include a bone screw and a yoke member for securing a rod in relation to the bone screw arrangement. Some arrangements further include a separate locking piece that operates in conjunction with the yoke member and bone screw to lock the bone screw in an angular position. The yoke member typically includes an upper pocket for receiving the rod. The separate locking piece is typically positioned within the yoke at a location below the upper pocket. The bone screw often includes a spherical head that seats within the separate locking piece. Prior to final fixation of the rod, the angle of the bone screw can be adjusted relative to the yoke. A securing piece (e.g. a threaded plug or ring) is secured to the yoke to lock the rod within the yoke, and to lock the bone screw at a particular angular position relative to the yoke via the locking piece.
The bone screw typically includes internal drive structure, such as an internal hex socket, formed in the head of the bone screw. The hex socket is adapted for receiving a wrench for use in driving the bone screw into a bone element. In use, the head of the bone screw is seated within the bottom region of the yoke with the hex socket facing upwardly so as to be accessible through the yoke. To fasten the bone screw to a bone element, a wrench or other tool is inserted downwardly through the yoke and into the internal hex of the bone screw. The bone screw is then driven into the bone element by applying torque to the internal hex. After the bone screw has been driven into the bone element, the rod is positioned within the yoke, and the securing member is mounted to the yoke to lock the rod within the yoke, and to lock the bone screw at a particular angular position relative to the yoke via the locking piece.
The size of a bone screw arrangement is important for minimizing the invasiveness of the surgical procedure. Yet, size is also important for maintaining the structural integrity of the arrangement when applying torque and other forces to the components of the bone screw arrangement. Often, in attempting to provide a less invasive arrangement by reducing the size of the arrangement's components, the structural integrity of the arrangement is jeopardized. For example, a common problem of smaller sized arrangements having internal hex sockets is that the internal hex socket strips or experiences excessive wear.
In the alternative, larger sized arrangements that can accommodate the greater torque loading are more invasive and create a greater risk to the patient. In addition, a larger internal hex formed in the bone screw head of the arrangement has limited accessibility. For example, the size of the internal hex can be no greater than the size of the opening provided in the yoke through which a drive tool, such as an allen wrench, is inserted. If the internal hex is maximized to accommodate as much torque loading as possible, problems arise in aligning the allen wrench with the internal hex. In other words, as the size of the internal hex increases, the more difficult it becomes to align a correspondingly sized tool with the internal hex due to a lesser amount of space and lesser tolerance for misalignment.
Moreover, when the bone screw is at certain angles relative to the yoke, the hex socket can be difficult to access. This is typically problematic in procedures involving the removal of a particular bone screw assembly, but the screw has been locked within the yoke at an angle where the hex socket cannot be readily accessed through the yoke.
In general, improvement has been sought with respect to such devices and arrangements, generally to better accommodate: ease of use, reliability, manufacture, and assembly; and to minimize invasiveness.